Operating speed and its deviations along tangent and curve segments of highways is one of themost common measures to evaluate the consistency of rural two-lane highways. Currently, thereare several models for estimating operating speeds on higher-speed rural two-lane highways,including the Design Consistency Module (DCM) of FHWA’s Interactive Highway SafetyDesign Model (IHSDM). The DCM includes operating speed models for evaluating designconsistency on rural two-lane highways. Through 2009, the DCM was limited to higher-speedhighways (posted speeds of 45 mph and higher). The goal of the research presented in this paperwas to extend the capabilities of the DCM by developing an operating speed model for lower10speed rural two-lane highways (i.e., those with posted speeds of 25 to 40 mph) using datacollected by FHWA for that purpose.To determine the operating speed profile along a highway, models to estimate the 85thpercentile speeds on tangents and curves were developed, and existing DCM models foracceleration and deceleration rates for transitions between “preferred speeds” on horizontaldesign elements were expanded to apply to lower-speed highways. This paper documents themodel development process and provides a case study to illustrate application of the model.
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